2-dimensional steering system for lidar systems

ABSTRACT

A light detection and ranging (LiDAR) system includes a rotatable polygon having a plurality of reflective sides including a first reflective side. The rotatable polygon configured to scan one or more first light signals in a first direction. The LiDAR system also includes a scanning optic configured to scan the one or more first light signals in a second direction different than the first direction. A first light source is configured to direct the one or more first light signals to one or more of the plurality of reflective sides of the rotatable polygon or the scanning optic. A first detector is configured to detect a first return light signal associated with a signal of the one or more first light signals. One or more optics are configured to focus the first return light signal on the first detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/634,661, filed Feb. 23, 2018, entitled “2-Dimensional Steering System for LiDAR Systems,” the content of which is hereby incorporated by reference for all purposes.

FIELD

This disclosure relates generally to laser scanning and, more particularly, to using a rotating polygon with one or more light sources and detectors.

BACKGROUND

Light detection and ranging (LiDAR) systems use light pulses to create an image or point cloud of the external environment. Some typical LiDAR systems include a light source, a pulse steering system, and light detector. The light source generates light pulses that are directed by the pulse steering system in particular directions when being transmitted from the LiDAR system. When a transmitted light pulse is scattered by an object, some of the scattered light is returned to the LiDAR system as a returned pulse. The light detector detects the returned pulse. Using the time it took for the returned pulse to be detected after the light pulse was transmitted and the speed of light, the LiDAR system can determine the distance to the object along the path of the transmitted light pulse. The pulse steering system can direct light pulses along different paths to allow the LiDAR system to scan the surrounding environment and produce an image or point cloud. LiDAR systems can also use techniques other than time-of-flight and scanning to measure the surrounding environment.

SUMMARY

In one example embodiment of LiDAR system, a rotatable polygon has a plurality of reflective sides including a first reflective side. The rotatable polygon scans one or more first light signals in a first direction. A scanning optic scans the one or more first light signals in a second direction different than the first direction. A first light source directs the one or more first light signals to one or more of the plurality of reflective sides of the rotatable polygon or the scanning optic. First detector detects a first return light signal associated with a signal of the one or more first light signals. One or more optics focus the first return light signal on the first detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The present application can be best understood by reference to the figures described below taken in conjunction with the accompanying drawing figures, in which like parts may be referred to by like numerals.

FIGS. 1-3 illustrate an exemplary LiDAR system using pulse signal to measure distances to points in the outside environment.

FIG. 4 depicts a logical block diagram of the exemplary LiDAR system.

FIG. 5 depicts a light source of the exemplary LiDAR system.

FIG. 6 depicts a light detector of the exemplary LiDAR system.

FIG. 7 depicts an embodiment of a signal steering system using a single light source and detector.

FIG. 8 depicts an embodiment of a signal steering system using two light sources and two detectors.

FIG. 9 depicts a portion of the scan pattern generated by the embodiment from FIG. 8.

FIG. 10 depicts components for implementing another embodiment of a signal steering system using a dispersion optic to scan pulses in a direction.

FIG. 11 depicts another embodiment of a signal steering system using three light sources and three detectors.

DETAILED DESCRIPTION

In the following description of examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the disclosed examples.

Some light detection and ranging (LiDAR) systems use a single light source to produce one or more light signals of a single wavelength that scan the surrounding environment. The signals are scanned using steering systems that direct the pulses in one or two dimensions to cover an area of the surrounding environment (the scan area). When these systems use mechanical means to direct the pulses, the system complexity increases because more moving parts are required. Additionally, only a single signal can be emitted at any one time because two or more identical signals would introduce ambiguity in returned signals. In some embodiments of the present technology, these disadvantages and/or others are overcome.

For example, some embodiments of the present technology use one or more light sources that produce light signals of different wavelengths and/or along different optical paths. These light sources provide the signals to a signal steering system at different angles so that the scan areas for the light signals are different (e.g., if two light sources are used to create two light signals, the scan area associated with each light source is different). This allows for tuning the signals to appropriate transmit powers and the possibility of having overlapping scan areas that cover scans of different distances. Longer ranges can be scanned with signals having higher power and/or slower repetition rate (e.g., when using pulsed light signals). Shorter ranges can be scanned with signals having lower power and/or high repetition rate (e.g., when using pulse light signals) to increase point density.

As another example, some embodiments of the present technology use signal steering systems with one or more dispersion elements (e.g., gratings, optical combs, prisms, etc.) to direct pulse signals based on the wavelength of the pulse. A dispersion element can make fine adjustments to a pulse's optical path, which may be difficult or impossible with mechanical systems. Additionally, using one or more dispersion elements allows the signal steering system to use few mechanical components to achieve the desired scanning capabilities. This results in a simpler, more efficient (e.g., lower power) design that is potentially more reliable (due to few moving components).

Some LiDAR systems use the time-of-flight of light signals (e.g., light pulses) to determine the distance to objects in the path of the light. For example, with respect to FIG. 1, an exemplary LiDAR system 100 includes a laser light source (e.g., a fiber laser), a steering system (e.g., a system of one or more moving mirrors), and a light detector (e.g., a photon detector with one or more optics). LiDAR system 100 transmits light pulse 102 along path 104 as determined by the steering system of LiDAR system 100. In the depicted example, light pulse 102, which is generated by the laser light source, is a short pulse of laser light. Further, the signal steering system of the LiDAR system 100 is a pulse signal steering system. However, it should be appreciated that LiDAR systems can operate by generating, transmitting, and detecting light signals that are not pulsed and/use derive ranges to object in the surrounding environment using techniques other than time-of-flight. For example, some LiDAR systems use frequency modulated continuous waves (i.e., “FMCW”). It should be further appreciated that any of the techniques described herein with respect to time-of-flight based systems that use pulses also may be applicable to LiDAR systems that do not use one or both of these techniques.

Referring back to FIG. 1 (a time-of-flight LiDAR system that uses light pulses) when light pulse 102 reaches object 106, light pulse 102 scatters and returned light pulse 108 will be reflected back to system 100 along path 110. The time from when transmitted light pulse 102 leaves LiDAR system 100 to when returned light pulse 108 arrives back at LiDAR system 100 can be measured (e.g., by a processor or other electronics within the LiDAR system). This time-of-flight combined with the knowledge of the speed of light can be used to determine the range/distance from LiDAR system 100 to the point on object 106 where light pulse 102 scattered.

By directing many light pulses, as depicted in FIG. 2, LiDAR system 100 scans the external environment (e.g., by directing light pulses 102, 202, 206, 210 along paths 104, 204, 208, 212, respectively). As depicted in FIG. 3, LiDAR system 100 receives returned light pulses 108, 302, 306 (which correspond to transmitted light pulses 102, 202, 210, respectively) back after objects 106 and 214 scatter the transmitted light pulses and reflect pulses back along paths 110, 304, 308, respectively. Based on the direction of the transmitted light pulses (as determined by LiDAR system 100) as well as the calculated range from LiDAR system 100 to the points on objects that scatter the light pulses (e.g., the points on objects 106 and 214), the surroundings within the detection range (e.g., the field of view between path 104 and 212, inclusively) can be precisely plotted (e.g., a point cloud or image can be created).

If a corresponding light pulse is not received for a particular transmitted light pulse, then it can be determined that there are no objects within a certain range of LiDAR system 100 (e.g., the max scanning distance of LiDAR system 100). For example, in FIG. 2, light pulse 206 will not have a corresponding returned light pulse (as depicted in FIG. 3) because it did not produce a scattering event along its transmission path 208 within the predetermined detection range. LiDAR system 100 (or an external system communication with LiDAR system 100) can interpret this as no object being along path 208 within the detection range of LiDAR system 100.

In FIG. 2, transmitted light pulses 102, 202, 206, 210 can be transmitted in any order, serially, in parallel, or based on other timings with respect to each other. Additionally, while FIG. 2 depicts a 1-dimensional array of transmitted light pulses, LiDAR system 100 optionally also directs similar arrays of transmitted light pulses along other planes so that a 2-dimensional array of light pulses is transmitted. This 2-dimensional array can be transmitted point-by-point, line-by-line, all at once, or in some other manner. The point cloud or image from a 1-dimensional array (e.g., a single horizontal line) will produce 2-dimensional information (e.g., (1) the horizontal transmission direction and (2) the range to objects). The point cloud or image from a 2-dimensional array will have 3-dimensional information (e.g., (1) the horizontal transmission direction, (2) the vertical transmission direction, and (3) the range to objects).

The density of points in point cloud or image from a LiDAR system 100 is equal to the number of pulses divided by the field of view. Given that the field of view is fixed, to increase the density of points generated by one set of transmission-receiving optics, the LiDAR system should fire a pulse more frequently; in other words, a light source with a higher repetition rate is needed. However, by sending pulses more frequently the farthest distance that the LiDAR system can detect may be more limited. For example, if a returned signal from a distant object is received after the system transmits the next pulse, the return signals may be detected in a different order than the order in which the corresponding signals are transmitted, thereby causing ambiguity if the system cannot correctly correlate the returned signals with the transmitted signals. To illustrate, consider an exemplary LiDAR system that can transmit laser pulses with a repetition rate between 500 kHz and 1 MHz. Based on the time it takes for a pulse to return to the LiDAR system and to avoid mix-up of returned pulses from consecutive pulses in conventional LiDAR design, the farthest distance the LiDAR system can detect may be 300 meters and 150 meters for 500 kHz and 1 Mhz, respectively. The density of points of a LiDAR system with 500 kHz repetition rate is half of that with 1 MHz. Thus, this example demonstrates that, if the system cannot correctly correlate returned signals that arrive out of order, increasing the repetition rate from 500 kHz to 1 Mhz (and thus improving the density of points of the system) would significantly reduce the detection range of the system.

FIG. 4 depicts a logical block diagram of LiDAR system 100, which includes light source 402, signal steering system 404, pulse detector 406, and controller 408. These components are coupled together using communications paths 410, 412, 414, 416, and 418. These communications paths represent communication (bidirectional or unidirectional) among the various LiDAR system components but need not be physical components themselves. While the communications paths can be implemented by one or more electrical wires, busses, or optical fibers, the communication paths can also be wireless channels or open-air optical paths so that no physical communication medium is present. For example, in one exemplary LiDAR system, communication path 410 is one or more optical fibers, communication path 412 represents an optical path, and communication paths 414, 416, 418, and 420 are all one or more electrical wires that carry electrical signals. The communications paths can also include more than one of the above types of communication mediums (e.g., they can include an optical fiber and an optical path or one or more optical fibers and one or more electrical wires).

LiDAR system 100 can also include other components not depicted in FIG. 4, such as power buses, power supplies, LED indicators, switches, etc. Additionally, other connections among components may be present, such as a direct connection between light source 402 and light detector 406 so that light detector 406 can accurately measure the time from when light source 402 transmits a light pulse until light detector 406 detects a returned light pulse.

FIG. 5 depicts a logical block diagram of one example of light source 402 that is based on a laser fiber, although any number of light sources with varying architecture could be used as part of the LiDAR system. Light source 402 uses seed 502 to generate initial light pulses of one or more wavelengths (e.g., 1550 nm), which are provided to wavelength-division multiplexor (WDM) 504 via fiber 503. Pump 506 also provides laser power (of a different wavelength, such as 980 nm) to WDM 504 via fiber 505. The output of WDM 504 is provided to pre-amplifiers 508 (which includes one or more amplifiers) which provides its output to combiner 510 via fiber 509. Combiner 510 also takes laser power from pump 512 via fiber 511 and provides pulses via fiber 513 to booster amplifier 514, which produces output light pulses on fiber 410. The outputted light pulses are then fed to steering system 404. In some variations, light source 402 can produce pulses of different amplitudes based on the fiber gain profile of the fiber used in the source. Communication path 416 couples light source 402 to controller 408 (FIG. 4) so that components of light source 402 can be controlled by or otherwise communicate with controller 408. Alternatively, light source 402 may include its own controller. Instead of controller 408 communicating directly with components of light source 402, a dedicated light source controller communicates with controller 408 and controls and/or communicates with the components of light source 402. Light source 402 also includes other components not shown, such as one or more power connectors, power supplies, and/or power lines.

Some other light sources include one or more laser diodes, short-cavity fiber lasers, solid-state lasers, and/or tunable external cavity diode lasers, configured to generate one or more light signals at various wavelengths. In some examples, light sources use amplifiers (e.g., pre-amps or booster amps) include a doped optical fiber amplifier, a solid-state bulk amplifier, and/or a semiconductor optical amplifier, configured to receive and amplify light signals.

Returning to FIG. 4, signal steering system 404 includes any number of components for steering light signals generated by light source 402. In some examples, signal steering system 404 may include one or more optical redirection elements (e.g., mirrors or lens) that steer light pulses (e.g., by rotating, vibrating, or directing) along a transmit path to scan the external environment. For example, these optical redirection elements may include MEMS mirrors, rotating polyhedron mirrors, or stationary mirrors to steer the transmitted pulse signals to different directions. Signal steering system 404 optionally also includes other optical components, such as dispersion optics (e.g., diffuser lenses, prisms, or gratings) to further expand the coverage of the transmitted signal in order to increase the LiDAR system 100's transmission area (i.e., field of view). An example signal steering system is described in U.S. patent application Ser. No. 15/721,127 filed on Sep. 29, 2017, entitled “2D Scanning High Precision LiDAR Using Combination of Rotating Concave Mirror and Beam Steering Devices,” the content of which is incorporated by reference in its entirety herein for all purposes. In some examples, signal steering system 404 does not contain any active optical components (e.g., it does not contain any amplifiers). In some other examples, one or more of the components from light source 402, such as a booster amplifier, may be included in signal steering system 404. In some instances, signal steering system 404 can be considered a LiDAR head or LiDAR scanner.

Some implementations of signal steering systems include one or more optical redirection elements (e.g., mirrors or lens) that steers returned light signals (e.g., by rotating, vibrating, or directing) along a receive path to direct the returned light signals to the light detector. The optical redirection elements that direct light signals along the transmit and receive paths may be the same components (e.g., shared), separate components (e.g., dedicated), and/or a combination of shared and separate components. This means that in some cases the transmit and receive paths are different although they may partially overlap (or in some cases, substantially overlap).

FIG. 6 depicts a logical block diagram of one possible arrangement of components in light detector 404 of LiDAR system 100 (FIG. 4). Light detector 404 includes optics 604 (e.g., a system of one or more optical lenses) and detector 602 (e.g., a charge coupled device (CCD), a photodiode, an avalanche photodiode, a photomultiplier vacuum tube, an image sensor, etc.) that is connected to controller 408 (FIG. 4) via communication path 418. The optics 604 may include one or more photo lenses to receive, focus, and direct the returned signals. Light detector 404 can include filters to selectively pass light of certain wavelengths. Light detector 404 can also include a timing circuit that measures the time from when a pulse is transmitted to when a corresponding returned pulse is detected. This data can then be transmitted to controller 408 (FIG. 4) or to other devices via communication line 418. Light detector 404 can also receive information about when light source 402 transmitted a light pulse via communication line 418 or other communications lines that are not shown (e.g., an optical fiber from light source 402 that samples transmitted light pulses). Alternatively, light detector 404 can provide signals via communication line 418 that indicate when returned light pulses are detected. Other pulse data, such as power, pulse shape, and/or wavelength, can also be communicated.

Returning to FIG. 4, controller 408 contains components for the control of LiDAR system 100 and communication with external devices that use the system. For example, controller 408 optionally includes one or more processors, memories, communication interfaces, sensors, storage devices, clocks, ASICs, FPGAs, and/or other devices that control light source 402, signal steering system 404, and/or light detector 406. In some examples, controller 408 controls the power, rate, timing, and/or other properties of light signals generated by light source 402; controls the speed, transmit direction, and/or other parameters of light steering system 404; and/or controls the sensitivity and/or other parameters of light detector 406.

Controller 408 optionally is also configured to process data received from these components. In some examples, controller determines the time it takes from transmitting a light pulse until a corresponding returned light pulse is received; determines when a returned light pulse is not received for a transmitted light pulse; determines the transmitted direction (e.g., horizontal and/or vertical information) for a transmitted/returned light pulse; determines the estimated range in a particular direction; and/or determines any other type of data relevant to LiDAR system 100.

FIG. 7 depicts an embodiment of a signal steering system (e.g., signal steering system 404 of FIG. 4) according to some embodiments of the present technology. Polygon 702 has ten reflective sides (sides 702A-702E are visible in FIG. 7) but can have any number of reflective sides. For example, other examples of polygon 702 has 6, 8, or 20 sides). In some embodiments, the polygon is wedge-shaped. For example, one or more reflective sides of the polygon are of a trapezoid shape. Polygon 702 rotates about axis 703 based on a drive motor (not shown) to scan signals delivered from a light source (e.g., via output 706, which is connected to a light source such as light source 402 described above) along a direction perpendicular to axis of rotation 703. One implementation of polygon 702 has about a 50 mm diameter and a 24 mm height or less than 75 mm and less than 50 mm, respectively.

Mirror galvanometer 704 is positioned next to polygon 702 so that one or more signals emitted from light source output 706 (e.g., a fiber tip) reflect off of mirror galvanometer 704 and onto rotating polygon 702. Mirror galvanometer 704 tilts so as to scan one or more signals from output 706 a direction different than the direction that polygon 702 scans signals (e.g., edges 704A and 704B tilt towards and away from polygon 702 about axis so as to scan pulses along a path that is parallel to the axis of rotation of polygon 702). In other example systems, other scanning optics besides a mirror galvanometer can be used in place of mirror galvanometer 704. One implementation of mirror galvanometer 704 (or a scanning optic) has about a 50 mm length and a 24 mm height or less than 75 mm and less than 50 mm, respectively.

In some examples, polygon 702 is responsible for scanning one or more signals in the horizontal direction of the LiDAR system and mirror galvanometer 704 is responsible for scanning one or more signals in the vertical direction. In some other examples, polygon 702 and mirror galvanometer 704 are configured in the reverse manner. While the example in FIG. 7 uses a mirror galvanometer, other components can be used in its place. For example, one or more rotating mirrors or a grating (with different wavelength pulses) may be used. The solid black line represents one example signal path through the signal steering system.

Light returned from signal scattering (e.g., when a light hits an object) within region 708 (indicated by dashed lines) is returned to rotating polygon 702, reflected back to mirror galvanometer 704, and focused by lens 710 onto detector 712. While lens 710 is depicted as a single lens, in some variations it is a system of one or more optics.

FIG. 8 depicts a similar system as depicted in FIG. 7 except a second light source is added that provides one or more signals from output 714. The light source for output 714 (e.g., an additional fiber tip) may be the same or different than the light source for output 706, and the light transmitted by output 714 may have the same or a different wavelength as the light transmitted by output 706. In one example system, a single 1 W laser source has enough power to support three outputs (e.g., three fiber tips) and additional outputs can be added by using a stronger laser source (e.g., a 4 W laser source). Using multiple light outputs can increase the points density of a points map without sacrificing the maximum unambiguous detection range of the system. For example, light output 714 can be positioned to transmit light at a different angle from output 706. Because of the different angles, light output from light output 706 is directed to an area different from light output from light output 714. The dotted line shows one example pulse path for pulses emitted from output 714. Consequently, one or more objects located at two different areas within a region can scatter and return light to the LiDAR system.

For example, the region 716 (the dashed/double-dotted line) indicates the region from which return signals from scattered signals returns to the LiDAR system. The returned light is reflected off polygon 702 and mirror galvanometer 704 and focused on detectors 712 and 718 by lens 710. Detectors 712 and 718 can each be configured to receive returned light from one of the outputs 706 and 714, and such configuration can be achieved by precisely controlling the position of the detectors 712 and 718 as well as the wavelength(s) of the transmitted light. Note that the same lens (or optic system) can be used for both detector 712 and 718. The offset (e.g., relative positions and/or relative angles) between outputs 706 and 714 causes the light returned to the LiDAR system to have a similar offset (e.g., similar relative positions and/or relative angles). By properly positioning detectors 712 and 718 based on the relative positioning and/or relative angles of their respect light source outputs (e.g., respective positions/angles of outputs 706 and 714) and by properly controlling the wavelength(s) of the transmitted light, the returned light will be properly focused on to the correct detectors, and each received light pulse at each detector can be a point in the points map. Therefore, compared to the system with only one output 706, the system with two outputs can maintain the same pulse repetition rate and produce twice the number of points or reduce the pulse repetition rate by half and still produce the same number of points. As a non-limiting example, a system with two light outputs can reduce the pulse repetition rate from 1 MHz to 500 kHz, thereby increasing its maximum unambiguous detection range from 150 meters to 300 meters, without sacrificing points density of the resulting points map. In some embodiments, a pulse repetition rate of between 200 kHz and 700 kHz or between 200 kHz and 2 MHz is used.

FIG. 9 depicts a point map from a first design. This design has two channels (e.g., two light source outputs and two light detectors) placed in a way that the exiting beams have an angle of 8 degree vertically. The scanned pattern has vertical overlap. The scanned range is +−56 degree horizontally and +12˜−20 degree vertically.

FIG. 10 depicts a point map from a second design. This design has two channels (e.g., two light source outputs and two light detectors) placed in a way that the exiting beams have an angle of 6 degree vertically. The scanned pattern has horizontal overlap (+−45 degree). The scanned range is +−67 degree horizontally and +12˜−20 degree vertically.

FIG. 11 depicts a point map from a third design. This design has three channels (e.g., three light source outputs and three light detectors) to increase point density. About 2.88 million points per second can be obtained by using 3 light source outputs and 3 light detectors. The resolution can be further reduced to 0.07 degree for both directions. The speed of the polygon can be reduced to 6000 rpm.

Various exemplary embodiments are described herein. Reference is made to these examples in a non-limiting sense. They are provided to illustrate more broadly applicable aspects of the disclosed technology. Various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the various embodiments. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process act(s) or step(s) to the objective(s), spirit or scope of the various embodiments. Further, as will be appreciated by those with skill in the art, each of the individual variations described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the various embodiments. 

1. A light detection and ranging (LiDAR) system comprising: a rotatable polygon having a plurality of reflective sides including a first reflective side, the rotatable polygon configured to scan one or more first light signals in a first direction; a scanning optic configured to scan the one or more first light signals in a second direction different than the first direction; a first light source configured to direct the one or more first light signals to one or more of the plurality of reflective sides of the rotatable polygon and the scanning optic; a first detector configured to detect a first return light signal associated with a signal of the one or more first light signals; and one or more optics configured to focus the first return light signal on the first detector.
 2. The system of claim 1 further comprising: a second light source configured to direct one or more second light signals, different from the one or more first light signals, to one or more of the plurality of reflective sides of the rotatable polygon and the scanning optic; and a second detector configured to detect a second return light signal associated with a signal of the one or more second light signals.
 3. The system of claim 2 wherein the one or more optics are further configured to focus the second return light signal on the second detector.
 4. The system of claim 2 wherein a relative position or a relative angle of the first detector to the second detector is based on a relative position or a relative angle of the first light source to the second light source.
 5. The system of claim 1, wherein the rotatable polygon and the scanning optic together scan the one or more first light signals into a first scan area and scan one or more second light signals into a second scan area different than the first scan area.
 6. The system of claim 5 wherein the first scan area and the second scan area partially overlap.
 7. The system of claim 6 wherein the first scan area and the second scan area vertically overlap.
 8. The system of claim 5 wherein the first scan area and the second scan area do not overlap.
 9. The system of claim 1, wherein the rotatable polygon and the scanning optic together scan the one or more first light signals into a first scan area and scan the one or more second light signals into the first scan area.
 10. The system of claim 2 wherein the one or more first light signals includes a first plurality of light pulses having a first pulse repetition rate, and the one or more second light signals includes a second plurality of light pulses having a second pulse repetition rate.
 11. The system of claim 10 wherein the first pulse repetition rate is between 200 KHz and 2 MHz, and the second pulse repetition rate is between 200 KHz and 2 MHz.
 12. The system of claim 10 wherein the first pulse repletion rate is about 500 KHz.
 13. The system of claim 2 further comprising: a third light source configured to direct one or more third light signals to one or more of the plurality of reflective sides of the rotatable polygon and the scanning optic; and a third detector configured to detect a third return light signal associated with a signal of the one or more third light signals, wherein the one or more optics are further configured to focus the third return signal on the third detector.
 14. The system of claim 1 wherein the scanning optic is a mirror galvanometer.
 15. The system of claim 1 wherein the scanning optic is a rotating mirror.
 16. The system of claim 1 wherein the scanning optic is a grating.
 17. The system of claim 1, wherein the one or more first light signals is one continuous light signal.
 18. The system of claim 1, wherein the one or more first light signals is a plurality of light pulses.
 19. The system of claim 1, wherein the first light source is configured to supply the one or more first light signals to the scanning optic before the one or more first light signals are supplied to the rotatable polygon.
 20. The system of claim 1, wherein the rotatable polygon is wedge-shaped. 